The present invention relates to solid catalysts and to the method of preparation. Specifically, the invention relates to the method of producing catalysts having a core which is usually catalytically inactive, with a thin outer shell bonded to the core and containing a catalytically active material. One particular embodiment of the invention includes the formation of an acidic alkylating catalyst having a catalytically inactive core and a thin outer shell containing the acid catalyst.
The use of solid catalyst comprised of small particles containing catalytically active materials on the surface thereof are desirable for a wide variety of chemical reactions. As one example, it would often be desirable in certain catalyzed chemical reactions which are historically catalyzed by liquid catalyst to replace the liquid catalysts with solid catalyst to simplify the process, and to reduce environmental concerns associated with such liquid catalyst. As one example, the production of highly branched hydrocarbons such as trimethylpentane for use as gasoline blending components for octane enhancement traditionally involves alkylation using conventional strong liquid catalysts such as hydrofluoric or sulfuric acid. The use of these liquid acid catalysts creates certain environmental concerns. With hydrofluoric acid, the concern is the possibility of the release of toxic vapors. With sulfuric acid, there is no acute toxic release problem, but there is the need to truck away and treat the waste acid which involves thermal decomposition and preparation of fresh acid. The transportation of the waste and fresh acid is closely regulated to prevent spills.
In view of the potential problems with the liquid acid catalysts such as used for alkylation, it is desirable to use less hazardous and toxic and more environmentally acceptable catalysts. Specifically, it is desirable to use solid catalysts rather than liquid catalysts. However, the use of solid catalysts at least for certain reactions such as alkylation has not been very successful. The main problem is the very short catalyst life which is sometimes measured in terms of hours, or possibly a few days.
One type of catalyst which holds promise for such reactions comprises a catalytically inert core which is covered with a shell comprised of, or containing, the catalytically active material. Such thin film or shell catalysts are not in themselves novel and have been disclosed in patents such as U.S. Pat. Nos. 4,394,251; 4,793,980; 4,427,577; 4,677,089; 4,378,308; 5,082,814; 5,200,382 and European Patent Application No. 323,735. These supported catalysts of the prior art are characterized by a number of factors which influence their usefulness. One is the penetration of the catalytic coating into the core when a porous core material such as alpha-alumina is used. It is well known to those skilled in the art that porous supports can be impregnated with a solution containing a catalyst precursor which fills the pores within the oxide support. This method is often referred to as the dry impregnation method or the incipient wetness method. Impregnation using slurries containing colloidal particles, i.e., a sol, using this prior art technique shows that there is substantial penetration of the sol into the substrate. This, as well as the coating techniques themselves, result to varying degrees in a non-uniform coating thickness. In addition, prior art preparation methods result in a range of pore diameters and in films of various thicknesses. The pore diameters within the film and the film thicknesses strongly influence the rate of diffusion of the reactants to the active sites in the pores and of the reaction products out of the film. As the pore diameters decrease and as the film thicknesses increase, the diffusion of reactants and products will lead to deleterious reaction products which foul the catalyst surface. This concept of catalyst deactivation has been recognized in the published literature. A high flux of reactants and products is necessary to obtain a high number of molecules reacted per unit of time for each active catalyst site and, more importantly, to suppress undesired reaction products which foul the surface. This is partially due to the long residence time within the intricate network of pores resulting in unwanted side reactions in conventional catalyst or in thin film catalysts of uncontrolled film thickness. Although it has been known that it would be desirable to concentrate the catalyst sites in a thin layer on the surface of a core particle, the techniques for forming such a uniform layer have not been satisfactory. Therefore, we have sought a method of catalyst preparation where it would be possible to prepare uniform thin films, or thin shells, of controllable thickness comprising catalysts or catalytic supports on a wide variety of substrates.